Electrical ratio meter



iJ.KNUDsEN ELECTRI CAL RAT IO `METER Filed sept. 15, 1942 2 Sheets-sheet l INVENTOR. v y KYIU kx). .v l HYWN' l Pelea-ine Yw. im

- UNITED f STATES PAT OFFICE Y l 7 2,391,168 f t l ELECTRICAL mnomnm Knud Knudsen, Warren, Ohio i o :implication september'iaisiaseriel No.4 ss.4 s 1 y v7 claims." (ci. 1v1-95)v I'his inventionrelates to a meter for indicat- I ing.thev ratioand direction of electrical currents One o! the principal objects is to provide a relageneral character in which it is possible to inany manner'disturbing or deiiecting the magnetic 7 and portioulerly to 'e meter of the direct current permanent'magnet type having moving, coils.

ys tall'- o r remove and replace the moving coil as t semblyas v(unit complete in itselLwithout inv` ilux through vthe air gap of the magnetic circuit. 'Another object is torprovide'a direct current going iiuxthrough v`a .'-"cofre"for actuating the de` fleeting coil'oi a" movingcoil assembly so that the entire iiux established in `the air gapoi the x f pement magnet is util d'enectively for opervating themoving coil.

Accrrelative obiectis tofutilize the same maior aux ileld 'tiret actuator the deneeting coil l:or j causing the restoring torque of the restoring coil.

- Another object is to provide such a ratio meter inllwhich the restoring coil is actuated by an externai electrical source'which has a proportional coul changes in this source.V

i kspeciiitg' object is to Provide' in. a high sensitivity'fmeter 5a' bearing-'screw ot'a new designv by which-the bearing is held in place emci'ently and securely and which is easily operated for installation, removal, a d replacement of the inev-l ing coilfassembly.

Another. object is to provide a meter of this character-in which V the electrical zero point. i. e.

`r meter which utilizes both the incoming and out-l thepointjat which nocurrent passes inv thev denecting coil. is at the approximate centeno! the indicating scale so that the amount oi ccm- -pensation that mustbe'made for temperature is reduced relative to what ls required in e straight line instrument formeasring currentsp and in which a rmeans in the forrn of ,a calibrating re-r sistor is providedjior maintaining a substantially -iixed ratio between the changes in conductivity otfythe deilecting coiland restoring coil reultimiv i'i'om changes in temperature, whereby variations in deflection oi' the defiecting coil. due

to heating 'of' the deecting coil, are substantially compensatedi f vAnother obiect is to providean instrument of this'eharact'er in which separate and independ* entadiustable Ameans are provided: one for setting' the instrument lto` indicate the electrical center at thecenter of the indicating scale. the

othermeansior calibra'tng the full scale, whereby the usual tedious cut, and vtry methods. o!

adjustment, due tofinterrelated adjustableimeans necessitated because the electrical center is outf side of the scale range; are eliminated.

`Another object is to provide a vmeter in which the magnetic assembly and the moving coilas` sembly each is complete in andof itself whereby the magnetic circuit'may vbe originally magnetized without the moving coil assembly being in" stalled, thus protecting the` delicate moving'fcoil assembly from any damage caused by the shock occasioned-during magnetization of the magnetic assembly. r Y

Another object is to'arrange the denectingcoil and restoringcoil relative to a common core'so that the riluill *is substantially equally available ior bothcoils and both gn be subjected to the same ilux at-all positions of movement. I

Another object vis to provide en instrument of this character which utilizes fewer parts than assembledmore readily f Other objects and advantages will becorneapparent from the following' specincation wherein reference isv made tothe 'drawings' infwhi'chf* heretofore ioundgneces'sary and-which may be j Fig. 1 is a'iront" elevation of a meterl embody-y ing the principles of ther4 present invention, as-

suming the meter to be positioned withthe iiidicating dial exposed upwardly: l ,f

Fig. 2 is a plan view of the'metervillustrated in .P ig. 1.' with the top bridge;l dial plate,y and coiled flexible connector removed for clearness in illustration. F18. f3 is a lcross sectional view taken on the llnel-3in1ligs.1nd2;x

Fis. 4 is a horizontal sectional view taken on alinellinllig.3:"v Fig. 5 is awiring diagram ofthe meter connected tor operation from a variable source of Vcurrent to be measured;

Fig. '6 is a longitudinal sectional view illustrating the Jeweled bearing' and mounting used in connection with'the present invention.

Ratio metersare utilized in'numerous appli` cations such as remoteindication of 'fuel oil pressure, hydraulic pressures, differences in' temn peratures, and anyothenpllienomena which, .dil

rectly or throughthe iterposition ofmechani-j cal or other means. causes; proportional varia- .t

tions in current due to;,chaige in resistance'. y

without-intending to limit thepresen't invention to the particular use shown, a 'preierred-embodiment oi'the invention .is illustrated lin, conlltliiiori with a remote pressure indicating sys- `teni for fuel oil pressure. -its use for other purposes being readily apparent therefrom.

Referring to Figs. 1 to 4. inclusive, the meter comprises generally a magnetic circuit assembly and a moving coil assembly, each of which is a complete sub-assembly in and of itself so that each sub-assembly can ybe assembled separately and apart from the other and then the two assembled together easily.

The magnetic assembly is best illustrated in' y Figs. 1 and 4 and comprises a permanent horseshoe magnet I for which generally channelshaped iron pole pieces 2 are provided. EThe pole pieces 2 are iixedly secured to a non-magnetic top plate 3 and thus held permanently` in fixed j relation with regard to each other and with their 1l 5,:bearingsforengagement with the pivots Il and bases snugly press fitted in contact with'theA in`` ner faces of the arms of the magnet I and opening toward, and aligned with, each other.

Secured to the top plate 3 is a soft iron core 4 which is generally in the shape of an annulus with its outer perimeter generally circular butv somewhat eccentric to the axis `of the central` opening so that more iron exists in the rear portion of the core to concentrate theliiuilrfinthev airKgap/ of the core. The core-I` has an 4integral extension I throughwhich the securing Lhe bolts 8 being of brass or other nonaterial. Interposed 'between the. soft in spac `relation axially andlaterally fromthe pole pieces' 2. It occupies the major portionof f the fspacewithin the limits of the pole` facesv 2..v4

4.` and top plate 3 are non-magnetic by which the core is definitely fixed re passed for securing the `core tothe Securedtoliie, top plate v,l by suiitabl,ef-,screws,r ,i is ano magnetic dial plate 8.` Mounted on the. topplate -.3,is a top brid e 9 which isof `non- ,a

magnetic material, such s brassorphenolic orf` other-,syntheti resin and which is spaced a detegra magnetic portions or spacers I I.y /Caraboye the ytop platerl'yr suitable ini secured to the upper pivot and extending therefrom so as to overlie the dial plete l when the coils are installed in operating position in the magnetic assembly is a pointer or indicator arm Il of non-magnetic materiel. suitable electrical connections between the coils and the external circuit are provided, as hereinafter explained, and suitable insulation is provided throughout, all in a well known manner.

The Vcore piece I, Yas mentioned, is generally inthe form of an annulus, but it isopen at the forwardportion to provide an air gap, as by spaced termini 4'.

On the upper and lower bridges are aligned ZlQof the moving coil assembly for supporting the moving coll assembly in the magnetic ileld. The-,bearings are.coaxia1 with an outer circular portion of the core piece e as will be described later.

The defiecting coil I5 is arranged with its axis at right angles'to and intersecting the upright pivotal'axie and' the restoring coil I1 extends at right anglesfrom the end ofthe deijiecting coil so as Ito swingy therewith about thepivotai axis.

'rheopening through the, dene'cting. eoiiis auch v thatfthe coil surounds and accommodates with slight clearancethecore piece, I, VThus, upon rotationdof the coil I5 about the` pivotsy I9 and 20, the deflecting `coil Ilswinlzs ajboutvthe core l, maintaining a yuniform spaced relation thereto.

' The restoring coil"fI1is of the proper size to be receivedthrough. the air .gap of the core piece 1 andhasA av passage therethrough such that the l coilaccommodates.with clearance first one and then'lthe other-ofthe' arms" ofthey core piece 4 as the deflectingcoil .l 5` is swung about the pivots I9 andI 20 in "opposite directionsffrom the air gap. Thus in allbuta yneutral position, the one 't0 illustrated, bothcoils areprovided kwithan effecriedbythe pole-pieces 2 and positioned below the also is completein' a deflectlng coil' I5 ight metalfrarne I is wound on a l-Is and '1111 be'.

opposiytkely .e ried counter A paced Tparall/el tivecoreso that the flux is most effectively utilized.

The moving coil` assembly maybe moved as a unit. intoplace inthe `magneticassembly and the pivots,l` I 9 and connected inproper relation, then ,the ,electrical `connections--ma,de.

Ingorde'r toavqid possible breakage of the rela- I tively small wiresof the coil due to repeated oscillations,..the terminals of V`thecoils ,which are to be connected .to the external circuit, as will hereinafterbe disclosed, are preferably connected to phosphor, `bronze coiledl leads or 1 hair springs which preferably exert little or no` resistance to f movement of the moving coil assembly. Each of thesegpringsis ,soldered `at one end to its particular terminal of ,the coils and itsother end is 'secured tothe propervtermlnal of the external circuit byl soldering or 'otherwise In the form illustrated, in which three1 leads to the external circuitare/hecessary, thehair springs, 6,0, i I, and

62 are utilized. The springs, of course, are electrically insulated from thedevice and electrically connected to their respective connectors B3, Il, and 65 `by`which they are connected to the external circuit. f Y, i

In 'order to provide van, effective and stable bearingfor thepivots I9 and 20 and ,at the same time provide for ready installation, detachment and dismounting of the coil assembly, re-

, movable bearings, such as illustrated inFig. 6, are used Ainthe top and bottom bridges 9 and I2,

respectively,` for receiving the pivots I9 and 20,

and are aligned/with each other and coaxial with the-outer circular surface 30 (Fig. 4) of the core piece ,4. illustrated'in Fig. 6 reach of these,,

or." thm-inner; end of the 'screw.;,lnterposed "betweenvithebearing and .thcfouterlend'of the s. suitable shoulder insicieineL GMWA Omte the 'bearing' isk aradially Alv eiui'an-` 4y y f ilanso) to. constrain the bearing from movement sible coil spring' 113'!V which Amay be' of screw'M--is/ provided with a pairofzlexpansion ll'ror: any -apmopriate plurality5 'oi slots,

stalled, is underf sumcient stressv to expand fthe phosphor bronze and`whi'ch1urg'e's y 'fthebear`ing-f.3l' ;against;'the shoulders-38.1 The Y t2, respectively, and a siniilarle'ad-in element is shown in Fig. l at vconnectedtothe hairsnring ll. O neor both adjusterar'ms can have suitable'f insulative mountingsl i! necessary to enable the j armsto actfasel'ectrical conductors.y The inner. turns oi the respective hair Asprings are con "necte'dto 'terminio'f the-v detlecting coils' l l and restoring (coil il in any suitablemannerLsuch as extend iorigitudinaiiyftiiereoilipart way of .softhat the screw may expandy or vbef.'v l asit*isSlelfvfted"5toforce,4 intoplace.: The spring 31,when inslottedr-'Irortionv ofthe screw radially or-to permit f itr tocontract radially` under 'outward' yieldingy pressure andthus hold itresilient1y1 but. tightly incontact withitscomplenientary'threadedeleholdingrthe screwf'inposition; a regu lator lock-Il hasinternal threadsv in engagement with the external threads of 'the 'screw 34"*'and is riveted or-otherwise heldfinplace'for rotational@ y adiustinenton the associated:bridge.y Distortion 'l lofthe oentralportionotthe regulator (screw I4) mq V 1 -,e utilizedftojassurethat jthe v regulatorv lock# *l ingfscrewremains inthe position. Thus I' f -thebearings of the pivots. I I-an'd N'canbe moved toward each 'other the proper .distance and ad- :usted l:or supporting -tiieiiioving lcoil iisiseiiimyV ywithtl'ie const hereo(` surrounding the 'ironcore 4 with substantially'equaltspac'es between theupf perfandlower faces-oi the core and 'the ii'ppel. i andflowef'sidesrortrie coilsl in su pci'iiitioris''oi'V thecoil unit or assembly.'Y

theinstrumentis'vv illustrated and a circuit is 'mgeo'ii l 1; andthe: u, and meines with thet'restorig coil il'forms one arm of thebridge.v

cient capable lof ychanging its' resistsncepwiin i cliana'eL in` temperature as in a` temperature indicator. Between. the termina1'4'l and the resistory "a calibrating resistor 5i is connectedin sliunted' relation around the restoring coilfll; Tliec'iiili fistf A resistor/T44 oisubstantially 'equal ohmic value pis connectedbetween the'te'rminals' 42 landg4yl. r- Acvali-batingfresistorv risfconnected between the galvan'orneter terminal 4l1oi' the bridgeand a dit f baite'rytemilnai unie' tenniriai u may 'coni necte'd,y to' che negative" side affine battery or source of direct ycurrer'it"and terminalj42 connec'ted to'theupositive side@ ofthe battery,v as shown.y .'Int'erposedbetween the terminals 43 and 4l`isan external'controlling variablesuchas a'`V4 f f variable' resistor 4Q vwhich may lhave anfadjust I f ingfarrnt operatedby oilfpressure''or-any'suit-I `able ineens to vary the resistor f49; Alternatively, Igxnayquse'a resistor of `high temperature co -el ne l I l' 3 brei-iu miriam from iiibiiiniiniy me temperatureco-lemcicntwire. or wiraths resistancooi which doesnot substantially change' with ferred tojand'shownat 60, ll and l2 inFigs 1' and 3. Pivoted adjuster arms 8 3 and I4 are indicatedinlllggfas connected tothe springs 6l and by connections'suggested in Figs. 1` and 3 but not yfullyshown. lIn the structure'thus illustrated. the hair springs aresoV wound that when no currentwhateveris'applied'to the device, the pointer or indieating arm 23 is at theleft of theindicatingdial and the vrestoring coil Il is to the' right Vof i the'.l airgap, termini 4', in the core as viewed in Fig.' 2.

Upon connection of the battery 'to the terminals 42 and 41, thek indicator moves to the central' positionont'hediaij in which position the restc'ring coil lies-in the-'air gap oi' the core 4, providing there is no current 4in the de'ilecting4 coil, this `being the electrical center or the instrument.' f

,'I'hereafteni with full negative currentr in theA deiiectinjgfcoil `il'the rindicator' moves t'o the left or directionand amount of unbalance of theWheatstonev bridge. By the use of thevtwo calibrating" resistors 4 8 and 51|, the adjustment f or'neutral position or electrical center of thescale 'can be ob'- tained withthe resistor 46, while', witli'theresistor 5I. adjustment for calibrating to the full scale can' be obtained, thus eliminating th f"prior tedious'eiii and ir'ymetiiods wnichrequir' dinar) i viidjustriienthad "to, be madesimultaneously in lsome manner because the electrical centerlwas,

suppressed 'and outside of thescale range. l

Hferetofo're non-temperature co-eilleient "wire resistors have been used and such resistors have'A been used in serieswith the deectingcoil so as to prevent a substantial increase in resistance in the deilecting c oil'circuitwitna consequent re# duction in current and error in indication., This f naturally reduces the, eillcenc'y' of the indicating devic'el In 'the present invention.' the Calibrating resistor ,fai/o1' zero'tem'p'er'ature' co-efllc'ient wire ris connected in shunt with the restoring coil Il k and insteadof being for the purposeof preventing an increase in the resistance of the deile'cting n coil, duev to increases in' ambient terripeiature,v is

used for the purpose `of decreasing the current in the restoring coil, in a direct'ratio to the decrease l i Ivo1' 'the defiectingj'coil current.' In'the form shown,

for example, when thej current'throu'ghthe defleeting coil is decreased due to an increase inv ambient temperature, 'acorres'ponding decrease in current'vtakes place in the restoring coil vin 'order-"to lr`ri' 'i'iiritia.in a correct pointer position. y Without the ea'librating" resistor S'I'anddue t0 the series connection of' the'resistor 45 'witht'he v restoring coil Il, the latter does not receive a due tothe increase of the resistance o! thefrey storing coil. Thusthe restoring effect of therestoring coil remains in a fixed ratio to the detlecting effect of the coil I with changes in ambient temperature. f In eiiect, therefore, thestructure operates ap proximately as though 4the deflecting coil were` subjected to the ambient temperature in thev usual manner and had a return spring which, in some manner was made operative so that its re storing torque r was decreased in a fixedrelation to the increase `of ambient temperature atany given time due to' higher resistance of the deflecting coil upon heating. Thus,` where the prior structure sought to reduce theeil'ects of heatingk of the derlecting coil and thus rendered the current less effective for operating the instru-f ment, in the present structure heat effect ispermitted to act uponthe restoring coil so as to reduce its restoring eilect and not' to resist. so greatly thetorque of the detlecting coil. l

As the moving coil assembly is deflected from the centered position, the restoring coil tends to restore the assembly to the center position with a force which is approximately proportional to the deflection in either direction from center and thus the instrument is in its trueelectrical sense l a `zero center galvanometen even though the position of the pointer and moving coll assembly is to one side of center when no voltage is applied. a Furthermore, compensation for temperature .A

errors has been greatly simplified by usingk a zero center instrument. Forexample, with Yno compensation, the percentage of Aerror encoun# have an electrical center suppressed below the v end rof the scale. Thus if a prior instrument hadk a 100 effectivescale as compared to the present instrument with a 100 'efl'ective scale, the prior instrument would have to be depressed about 50. Therefore the distance fromelectrical center to maximum deflection would be 150, whereas in the present instrument it is only 50. As a result, with no compensation there would be three times as muchA error in the suppressed center instru-` ment as in the present instrument at maximumy deflection. Therefore, compensation can be accomplished in the present instrument -by the use of the appropriate non-temperature co-efllcient resistor with a restoring'coil, whereas such would not be as eiectivein the prior suppressed tive positions of the parts, and not finite positions. l

for convenience in description. The various resistors, including 51| may be located remote from the other parts of the instrument, and for clearness, have been omitted in Figs. 1 to .4. as their physical location is, as is well known, a matter of convenienceorchoice.

5i lI claim:Y i

1. n `instrument of thedirect .current moving` coil type and comprising a horizontally disposed horseshoe'magnet having its arms extending forwardly, a pair ofgenerally channel-shaped .10 pole pieces in fixed relation to the arms of the magnet. respectively, with their 'sides extending horiaontallmsaid pole pieces opening toward and being in spaced relationto eachother to define a major flux path,a horlzontalgenerally` circularcore, `means extending rearwardly-from the core and nxedlysupportingthe core between the pole piecesin-spaced :relation thereto and with portions of rthe core extending part way within the open-.channels ofthe pole plecesya pair of relatively. axially adjustable bearings coaxial with the core and arranged one above and onebelow the core, pivot [means mounted in said bearings, a detlecting coil carried by the pivot means with its axis horizontal and intersecting the axis of the pivot means at right angles, said coil having an axial passage of greater height and length than the height and outerdiameter'respectively of the core for accommodating the core within .the coil with clearance in oscillated positions of the coil, said coil lextending between the core and pole pieces with clearance relative to the pole pieces, and restoring means for the coil rigid therewith at righty angles thereto and adapted to encircle arm portions oi' the core, said coil and restoring means being removable as a unit from the magnet and core assembly without disturbing the magnetic ilux circuit of saidassembly.

2. An instrument according to claim 1 and further Acharacterized inthat saidfcore has a cen- 40 tral aperture and an airgap in its forward p0rtion and said restoring meansis a restoring coil which is carried ,by the pivot means and has its axis horizontal and spaced forwardly from the pivotal axis and .said restoring coilhas a passagetherethrough which accommodates respecrestoring coil oscillates about the .pivotal axis, andA the saidrestorlngcoil is of less width axially than saidpair gap, ,whereby the assembled coils and .50 pivot means may be removed forwardlyi'rom the devlcewithout disturbing the magnetic ,circuiti 3.` An electrical instrument of the moving coil type, comprising a magnet having opposed chan-l y nel shaped pole pieces with the flanges ofthe t5 channels facing each other and spacedapart in the same plane, a magnetic corecentrally located between said `pole pieces andhaving circular ex ternal peripheral surfaces uniformly spaced from the webs of the channelst in ythe common plane l thereof 'and providing spaced arms directed toward each other, a coilassembly including a deflecting coil pivoted Aon anaxisgenerally centrally of the core, said coil-being looped about the core and operatively related to the channel flanges so that opposite sides vof the c oil remain in substantially uniform `cutting relation to flux lines passingy between the flanges as .the coll "assembly is turned .through anarc of at least 90, said coilassemblyalso including a restoring coil f'lxed to the deflecting coil normal `thereto and receivable between thefarms of the core so as. to eni circle said arms in such turned positions of the coil assembly, said arrangement enabling the coils to be assembled as a unit into operativevrelatlony ship to the pole pieces and core withoutdisturbtively, the said armportions of the core as the le l sin ing the magnetic circuit` afforded by the magnet v and core.

ywith an upright axis, the core being centrally located between said pole pieces and having its external peripheral surfaces lying between said flanges and spaced therefrom and from the webs ofthe channels in the common plane -thereof and providing spacedv arms directed toward each other. forwardly from said axis, the webs of the channels being spaced apart from each other a' distance greater than the width of the core throughout those portions of the webs which are forward of said pivotal axis, a coil assembly including a defiectingv coil having a pivotal axis generally parallel to the axis ofthe aperture, said coil-being looped about the core in spaced re-` lation thereto and to the channels and normally occupying a neutral position in whichits axis is normal to its pivotal'axis and extends in said fore and aft direction, the core and channel shaped pole pieces being related to render the i'iux path substantially uniform coextensive with the outermost limit of the path generated by rotatingthe deiiecting coil through at least 45 n a opposite directions from said neutral position, said coll assembly also including 'a restoring coil fixed to the `deiiecting coil normal thereto and receivable between the arms of the core so as to en circle said arms in turned positions of thecoil assembly, said arrangement enabling thek coils to be assembled as a unit into operative relationship to the pole pieces and core without disturbing the magnetic circuit afforded by the magnet and core.

-5. In an instrument of the direct current moving coil type, a magnetic assembly complete in and of itself and including a pair of spaced pole faces and a core piece in ilxed spaced relation thereto in the maior iiux path therebetween, a

, and core dening a coil receiving space which a neutral position of saiddeiiecting c0il,^said pole pieces and core being shaped relative to each other to render the maior ilux path substantially coextensive with the outermost limits of the path generated by rotating the deilection coil through an angle of at least 45 in each direction from its neutral position, whereby said coil assembly is operatively related to the magnetic assembly in a manner to enable uniform scale deectionV through an arc of 90 and greater, and means to retore the moving coil assembly to starting posi 0n.

6. In an electrical instrument of the direct -eurrent moving coil type. a magnetic assembly com-` prising a permanent magnet, a pair of laterally spaced pole pieces, a core between and in xcd spaced relation to the pole pieces. said pole pieces comprises the major i'iux path and which is open at the front, relatively axially adJustable upper and lower axially aligned bearings above and be-v low the pole pieces and in ilxed lateral relation thereto, a 4moving coil assembly including a deiiecting coil and a restoring coil, upper and lower axially aligned pivots ilxedly secured thereto and receivable in the bearings, respectively, the axis of the deilecting coil being at right angles to the pivotal axis, and normally extending forwardly across the major flux path in a neutral position of said deiiecting coil, said core and pole pieces being shaped relative .to each other to ren der the maior flux path substantially uniform coextensive with the outermost limits of the path generated by rotating the deilecting coil through an kangle of at least 45 in each direction from its neutral position, said deflecting coil having a central passage and being receivable into said 40 into space'd relation to the pole pieces and core moving coil assembly complete in and of itself a and including a deilecting coil and pivots therefor and. iixedly secured thereto, thedeflecting coil beingadapted to lie within the major flux path between the pole pieces andcore in spaced relation tothe core and pole pieces and with its pivotal axis'normal to the maior nux path, and being receivable in slid ilux path and removable ytherefrom while the coil is in the moving coil assembly and while the magnetic assembly is inv assembled condition, bearings for the pivots operable to be engaged with the pivots of the moving coil assembly, respectively, and to oscillatably support the assembly with` the deii'ecting coll d in said flux path and in spaced relation to the pole piecesand core, said denecting coil having its axis normally extending at right angles to its pivotal axis and lying across the maior iiux path when so received, whereby said coil assembly is operatively related tothe magnetic assembly in a manner to enable uniform scale deflectionv through an angle of and greater.

7. An instrument according to claim 6 and fun ther characterized in that the restoring coil is fixedly carried by the moving coil assembly, the axis of the restoring coil being at right angles to and .spaced from the a/xis of the pivots and at right angles'tmthe anis koi' the deiiecting coil,

said core having acentral aperture and an airy gap at its forward portion connecting therewith' and dividing the core into two integral arm portions between the ends of which the restoring coil maybe passed without disturbance of the magnetic circuit. said restoring coil having a passage therethrough and surrounding the core for accommodating the arm portions of the core in spaced relation thereto when the coil assembly is swung about its pivotal axis out of neutral position.

KNUD J. 

